feat(ui): import comprehensive UX system from beads

Import beads' UX design system into gastown:

- Add internal/ui/ package with Ayu theme colors and semantic styling
  - styles.go: AdaptiveColor definitions for light/dark mode
  - terminal.go: TTY detection, NO_COLOR/CLICOLOR support
  - markdown.go: Glamour rendering with agent mode bypass
  - pager.go: Smart paging with GT_PAGER support

- Add colorized help output (internal/cmd/help.go)
  - Group headers in accent color
  - Command names styled for scannability
  - Flag types and defaults muted

- Add gt thanks command (internal/cmd/thanks.go)
  - Contributor display with same logic as bd thanks
  - Styled with Ayu theme colors

- Update gt doctor to match bd doctor UX
  - Category grouping (Core, Infrastructure, Rig, Patrol, etc.)
  - Semantic icons (✓ ⚠ ✖) with Ayu colors
  - Tree connectors for detail lines
  - Summary line with pass/warn/fail counts
  - Warnings section at end with numbered issues

- Migrate existing styles to use ui package
  - internal/style/style.go uses ui.ColorPass etc.
  - internal/tui/feed/styles.go uses ui package colors

Co-Authored-By: SageOx <ox@sageox.ai>

internal/doctor/identity_check.go

@@ -9,19 +9,19 @@ import (
 )
 
 // IdentityCollisionCheck checks for agent identity collisions and stale locks.
-type IdentityCollisionCheck struct{}
+type IdentityCollisionCheck struct {
+	BaseCheck
+}
 
 // NewIdentityCollisionCheck creates a new identity collision check.
 func NewIdentityCollisionCheck() *IdentityCollisionCheck {
-	return &IdentityCollisionCheck{}
-}
-
-func (c *IdentityCollisionCheck) Name() string {
-	return "identity-collision"
-}
-
-func (c *IdentityCollisionCheck) Description() string {
-	return "Check for agent identity collisions and stale locks"
+	return &IdentityCollisionCheck{
+		BaseCheck: BaseCheck{
+			CheckName:        "identity-collision",
+			CheckDescription: "Check for agent identity collisions and stale locks",
+			CheckCategory:    CategoryInfrastructure,
+		},
+	}
 }
 
 func (c *IdentityCollisionCheck) CanFix() bool {